Torque shop: Worn out bushings cause vibrations

What can cause vibrations that become most noticeable while driving and increase when accelerating hard?

If the engine behaves as usual and your wheels and tyres are in good shape, then the vibrations are due to some fault in the drive train. Vibrations that feel as if the whole car body is “rattling” result from a breakdown in vibration isolation.

Very few exceptions aside, a car’s engine and transmission – whether automatic or manual – are assembled rigidly together. When installed in the car’s chassis, the engine-gearbox unit is isolated from the body by specially designed vibration absorbers at each mounting point.

Depending on the car, the most commonly used absorbers are made from rubber or polyurethane, sometimes even liquid-filled for enhanced vibration damping.

Without the shock-absorbing interface, any vibration from either engine or transmission will be transmitted to the body. It is possible that the vibrations – which are almost invisible to the naked eye – could be amplified by the body panels or chassis elements, resulting in extreme discomfort to the car’s occupants.

You may not hear or feel the vibrations when the car is idling at a standstill, but they will be obvious when the car is cruising and especially when accelerating.

Engine and gearbox mountings naturally wear down with age and the rubber or synthetic material they are made from will harden or break over time, losing all the original vibration-absorbing property.

Continuing to use a car with worn or collapsed engine mountings will eventually cause other parts to suffer as a result of the high frequency physical disturbances.

Other less severe but nonetheless disturbing vibrations are from power transmission components, specifically the drive shafts. With rear-wheel-driven cars, the long shaft connecting the engine in front to the differential at the rear can become unbalanced due to regular wear and tear of the universal joints or constant-velocity couplings. So, too, can the shafts that transmit drive from the differential to the rear wheels. The same sort of imbalance in drive shafts can afflict front-wheel-drive cars too.

Drive train vibrations are almost always a result of wear and tear, with engine and transmission mountings heading the list. These vibration absorbers have a fixed life span and should be the first items to be replaced before moving on to the drive train components only if the vibrations persist.